Final answer:
The President can call a special session of Congress when a serious problem arises but Congress has adjourned for the session.
Step-by-step explanation:
When a serious problem arises but Congress has adjourned for the session, the President can call a special session of Congress. This power allows the President to convene Congress outside of its regular session in order to address urgent matters that require legislative action. By calling a special session, the President can gather lawmakers to discuss and potentially pass new laws or take necessary actions to address the serious problem at hand.